Unlike most conventional metallic superconductors, oxide high critical temperature superconductor ("HT.sub.c S") materials (i.e. materials which exhibit superconducting characteristics at temperatures above liquid nitrogen temperature of 77 degrees Kelvin) have a large normal state resistivity. Typical resistivities of YBaCuO material at 100 degrees Kelvin (".degree. K") are of the order of 100 .mu..OMEGA.-cm, which is at least one order of magnitude higher than that of metallic superconductors. The large difference in resistance between the normal state and the superconducting state is suitable for making high critical temperature ("T.sub.c ") superconducting switches that are compatible with many kinds of electronic circuits.
If the temperature of a HT.sub.c S material exceeds that material's T.sub.c, then the material remains in a "normal" state in which it exhibits a large resistivity to an applied current. Similarly, if the current applied to a HT.sub.c S material exceeds the material's critical current ("I.sub.c "), then the material will also remain in the "normal" state aforesaid. In the normal state, an electrical current passing through the HT.sub.c S material establishes an electrical voltage across the material, due to the normal resistance exhibited by the material. But, if the temperature of a HT.sub.c S material does not exceed that material's T.sub.c ; and, if the current applied to the HT.sub.c S material does not exceed that material's I.sub.c, then the material switches to the superconducting state in which it exhibits essentially zero resistance to an applied current. In this application, the term "superconductor resistor" is used to refer to HT.sub.c S materials which are capable of exhibiting dual resistance states as aforesaid.
A HT.sub.c S material can be switched between the normal and superconducting states by modulating the current applied to the material. This is the basic principle of operation of a current controlled superconducting switch ("CCSS"). If the current applied to a CCSS device is less than I.sub.c, then the device is superconductive with zero resistance, and no voltage is developed across the device (i.e. the switch is "off"). But, if the current applied to the device exceeds I.sub.c, then the device switches to the normal resistive state and a voltage is developed across the device (i.e. the switch is turned "on"). A CCSS device can accordingly be constructed by electrically connecting a HT.sub.c S element in parallel with a fixed (i.e. non-superconducting) resistor element. An input current signal can be used to control the device's switching behaviour , with the voltage developed across the fixed resistor representing the output. When the input current signal is below I.sub.c of the HT.sub.c S element, all of the current flows through the HT.sub.c S element and no current flows through the fixed resistor so the voltage output across the fixed resistor is zero (i.e. the switch is "off"). If the input current signal exceeds the HT.sub.c S element's Ic, then the current is divided between the two circuit elements and a voltage develops across the fixed resistor (i.e. the switch is turned "on").
The present invention discloses how such CCSS devices can be adapted to the fabrication of logic gates, circuit breakers, or analog to digital converters, all of which are useful in the construction of low temperature digital electronic devices.